The present invention relates generally to the bottom construction of industrial processing furnaces, such as metallurgical blast furnaces, and more particularly to a water cooled furnace bottom construction having a fluid-impervious seal for preventing internal gases and liquids from escaping outside and for preventing external gases and liquids from entering the inside of the furnace.
The blast furnace is a large, shaft-like, metallurgical reactor having an outer steel shell lined with refractory material. In the blast furnace, iron-bearing ores are chemically reduced to form pig iron. Impurities in the ores form slag with flux added to the furnace. Hot air, at an elevated temperature and pressure, is blown into the side of the furnace near its base to form reducing gases and to provide heat which melts the iron and slag. The reducing gases, because of the pressure differential up the shaft of the furnace, tend to mainly rise through the furnace and simultaneously heat and reduce the iron-bearing materials which are added from the top.
The hot metal and slag are retained in the refractory-lined lower section of the furnace (the hearth) until they are removed during a "tap" of the furnace. Over a period of time, the molten metal and slag accumulating in the hearth tend to react with and erode the refractory hearth lining, and, as a result, there is a decrease in the lining thickness at the bottom of the furnace interior, both around the side walls and above the hearth base.
Until recently, prior art furnaces had relatively small hearth diameters, and the hearth refractories could be sufficiently cooled by placing water cooled elements, called staves, around the periphery of the furnace hearth, i.e., between the steel plate and the refractory lining. The refractory lining was permitted to erode during furnace operation, and the hearth refractories had to be completely replaced one or two campaigns (a campaign comprises a period of furnace operation for several years). Because there was no cooling of the furnace bottom, i.e., no "under-hearth cooling," the ultimate vertical erosion of the hearth refractory, at the center of the hearth, was normally quite severe, and care had to be taken to ensure that the refractory hearth was relined before the molten metal eroded completely through the hearth refractory at the furnace bottom. Such erosion could cause tremendous physical damage and create a serious safety hazard.
In recent years, the hearth diameter of blast furnaces has increased, and ceramic refractories, previously used to line the hearth, have been replaced to a large extent with carbon refractories. As the hearth diameter increases, it becomes more difficult to withdraw heat from the hearth through the side walls and, as a result, erosion at the center of the hearth into the furnace bottom becomes more significant. In order to decrease this erosion, the underhearth in most large furnaces is cooled in some manner. For example, located below the refractory blocks which compose the hearth are cooling ducts through which a cooling liquid, such as water, is circulated. These cooling ducts rest on a concrete base and are embedded in a material called "ramming mix" which is rammed into place around and over the cooling ducts after the latter have been emplaced. The refractory blocks rest on the ramming mix, usually with a layer of leveling tiles between the blocks and the ramming mix.
It has been common practice to place a fluid-impervious seal, composed of welded steel plate, under the carbon refractories and above the cooling pipes or ducts. Normally, the steel plate is about 1/2 inch thick, and the seal is very rigid once it is in place. Also, it was common practice to install the ramming mix around the cooling pipes or ducts and between the concrete base and steel plate before the steel plate was emplaced. In the process of welding the component parts of the steel plate in place, the steel warped, causing air gaps between the steel plate and the underlying ramming mix. Each such air gap acts as an insulating layer; and this significantly decreases the overall thermal conductivity in the underhearth and, hence, decreases the flow of thermal energy downwardly from the carbon blocks in the hearth base ultimately into the cooling ducts in the furnace bottom. This, in turn, increases the erosion of the carbon blocks.
Carbon has a higher thermal conductivity than ceramic refractory material and will erode to a lesser depth than ceramic material, for a given set of cooling conditions. It is important that the cooling effect of the cooling ducts in the furnace bottom be manifest to as high a level as possible when carbon blocks are used for the hearth base. As noted above, air gaps between the seal plate and the ramming mix retard this cooling effect, lower the level of the cooling effect and increase the depth to which the carbon blocks will erode.
For example, in a blast furnace having a 45-foot diameter hearth with 116 vertical inches of carbon blocks in the hearth base and no air gaps, the carbon blocks of the hearth were calculated to erode away at the hearth center to a depth of only 38 inches. However, if a 1/8-inch air gap is formed below the entire steel plate and above the ramming mix, the carbon blocks at the center can be expected to erode to a depth of up to 73 inches, so that an additional 25 to 35 inches of carbon refractory material would be lost compared to a furnace bottom with no air gaps. Other calculations with deeper air gaps have shown that additional inches of carbon would be lost.
If a hearth base erodes too deeply during a campaign, it must be replaced when the furnace is shut down for a relining job at the end of a campaign. Replacement of the hearth base is costly in that the carbon blocks are composed of relatively expensive, high quality carbon and the down time for the furnace is increased. Accordingly, it is important that the carbon refractory hearth be usable for many furnace campaigns with a minimum amount of erosion in order to save costs and increase furnace operating time.